1. Field of the Invention
The present invention generally relates to screens which convert incident radiation, such as X-rays, into spectral emissions such as visible light. More particularly, the present invention relates to a radiation intensifying screen having two luminescent material layers with different spectral emission wavelength ranges and different emission maximum wavelengths separated by a reflective-transmissive layer which reflects the spectral emissions emanating from one of the luminescent material layers and allows spectral emissions from the other layer to pass therethrough. The configuration of the screen increases spatial resolution for a given screen speed.
2. Description of the Prior Art
X-ray intensifying screens have been widely used in medical and industrial imaging systems. An intensifying screen is a device that absorbs incident x-ray radiation and converts the incident radiation energy into spectral emissions of predetermined wavelengths. The radiation absorbing, spectral emitting material in an intensifying screen is typically a phosphor. The wavelengths of the spectral emissions from most phosphors used in intensifying screens are typically in the visible portion of the electromagnetic spectrum. The spectral emissions from the phosphor is received by a detector (such as radiographic film) which is responsive to the wavelengths of the spectral emissions to form an image of an object that has been subjected to the incident radiation.
One of the common objects of medical imaging is to maximize image contrast and spatial resolution while minimizing radiation dose to the patient. Spatial resolution is related to the ability of an imaging system to reproduce an image of an object faithfully. Generally, conditions that increase the intensification factor or speed of the screen typically reduce its spatial resolution. Speed or intensification factor increases can be obtained by increasing the thickness of the radiation absorbing, luminescent layer and by using relatively larger phosphor crystals. Resolution can generally be improved by using smaller phosphor crystals and thinner radiation absorbing, luminescent layers.
The intensification factor of screens can also be increased by using a reflective layer adjacent to the radiation absorbing, luminescent or phosphor layer. Referring to FIG. 1, a conventional X-ray intensifying screen 10 of the reflective type typically comprises a protective layer 12, a radiation absorbing, luminescent layer 14, a reflective layer 16 and a backing layer 18. Incident X-ray absorption at locations A and B in the radiation absorbing, luminescent layer produces spectral emissions in the visible light wavelength range in the layer which emit isotropically through the layer. Spectral emissions in the form of visible light A', B' emitted towards the protective layer have a relatively short path before emerging from the screen, thereby producing image information with relatively high spatial resolution. On the other hand, spectral emissions in the form of visible light A", B" emitted toward the backing layer may be reflected at the reflective layer and, if so, then exits the screen from the protective layer side. As compared to the light photons A', B' emitted towards the protective layer, the reflected light photons A", B" travel a longer path in the phosphor and generally have a large lateral dispersion from their emission sites. The longer lateral dispersion results in reduced spatial resolution.
As those skilled in the art will appreciate, the purpose of the reflective layer in an X-ray intensifying screen is to increase the speed of the luminescent screen which may enable suitable image contrast at a lower radiation dosage. Although this reflective layer nearly doubles the amount of visible light that can emerge from the screen, it does so at the expense of screen spatial resolution. In some applications such as mammography where high resolution is required, the reflective layer is not used. This improves the screen's spatial resolution at the expense of screen speed. As a result, high radiation doses to the objects (e.g., patients) being imaged are generally required to get adequate image contrast.
It would be desirable to increase the speed of an intensifying screen without severely degrading spatial resolution, Stated in the alternative, it would be desirable to increase the speed of an intensifying screen of a given spatial resolution or increase the spatial resolution of an intensifying screen of a given speed.